1. Field of the Invention
This invention is in the field of cryogenic processing systems.
2. Description of the Prior Art
The problems caused by water or other materials freezing in refrigeration equipment have long been recognized. For example, in U.S. Pat. No. 167,181, the problem of the clogging action of pure ammoniacal gas when such gas meets oil or other lubricating materials in icemaking machines or refrigerators is pointed out. It is pointed out that a viscous deposit which coats and clogs the pipes, and often caused partial or complete stoppages was often the result. To solve this problem, this patent teaches that the addition of 5 to 10% of benzene, or other hydrocarbon vapor, neutralized this problem.
U.S. Pat. Nos. 2,163,899 and 2,163,900 both deal with a similar problem in refrigeration equipment employing hydrocarbon halide refrigerants. These patents teach that certain organic ether compounds and organic acetate or ketone compounds, respectively, could be added in amounts of up to 10% to prevent the formation of ice crystals or to remove ice crystals already formed in hydrocarbon halide refrigerants.
Methanol has also been used in hydrocarbon gas streams to prevent hydrate and ice formation. An example of the use of methanol in such systems is described in U.S. Pat. No. 3,644,107 wherein the methanol is first conditioned by vaporizing it, and subsequently injected into such hydrocarbon streams for this purpose.
Freeze-out of a gas component is particularly a problem in systems wherein the gas mixture will be subjected to extremely low temperatures, e.g., cryogenic temperatures. Such temperatures are encountered, for example, in the separation or purification of certain gas mixtures by total liquefaction of the gas mixture. For example, a method for purifying a natural gas stream rich in carbon dioxide is disclosed in U.S. Pat. No. 3,306,057. In this method, a natural gas mixture containing a substantial amount of carbon dioxide is purified by a process of: (a) cooling the compressed stream in a system which includes a first heat exchanger charged with cold carbon dioxide and condensing it to a liquid; (b) fractionating the liquid to yield substantially pure liquid carbon dioxide and a methane-rich gas; (c) partially condensing the methane-rich gas in a second heat adsorber charged with cold carbon dioxide, recompressing, chilling and expanding the gaseous faction to separate out contaminating carbon dioxide as a solid; and (d) combining the solid carbon dioxide with the liquid bottoms from the fractionator reboiler and using the combined medium as coolant in the aforesaid heat adsorbers.
Although freeze-out of a contaminant from a gas stream has almost universally been considered a deleterious phenomenum, the teachings of U.S. Pat. No. 3,885,939 seem to suggest that contaminant freeze-out can be used to advantage. In this patent, a cryostat flow control in which the refrigerant flow rate is controlled by the addition of a contaminant or foreign fluid to the refrigerant is described. After initial cool-down, the contaminant, having a higher solidification point than the refrigerant, will solidify in the cryostat and cause partial or complete refrigerant flow stoppage. When the refrigerant flow is reduced or stopped, refrigeration slows or ceases with a resultant rise in cryostat temperature, which in turn then melts the solidified contaminant. Refrigerant flow will then resume until the temperature is again reduced to freeze-up or solidify the refrigerant contaminant.
Despite the teachings of U.S. Pat. No. 3,885,939, contaminant freeze-out is a very serious problem in most cryogenic systems and various methods have been devised to deal with this problem. Thus, in U.S. Pat. No. 3,282,059, an apparatus for dealing with this problem in the liquefaction of natural gas containing carbon dioxide and water vapor as impurities is taught. The solution taught by this patent is the provision of two parallel heat exchangers which allows the gaseous stream to be circulated to a second heat exchanger after the first has become coated or caked by the adherence of solid impurities. The clogged heat exchanger is then flushed with a natural gas stream which is then refed to the feed stream.
In U.S. Pat. No. 3,793,846, a Stirling cycle refrigerator apparatus is disclosed which includes a contaminant adsorber positioned in the cold portion of the refrigerator and connected to selectively transmit gas to the cold side of the refrigerator so that the regenerator is purged with relatively warm gas from the adsorber.
The problems caused by contaminant freeze-out are particularly severe in the cryogenic processing of a gaseous helium stream. In a typical helium feed stream, there are many impurities, such as neon, nitrogen, hydrogen, methane, ethane, propane, butanes, pentanes, hexanes, carbon dioxide and heavier hydrocarbons. Some of the hydrocarbons, such as methane, for example, are typically removed in the preprocessing of the helium stream prior to its final purification which involves very low cryogenic temperatures to remove remaining contaminants such as carbon dioxide. Unfortunately, reducing the level of hydrocarbons in the preprocessing often aggravates CO.sub.2 freeze-out problems since some hydrocarbons can act as solvents for CO.sub.2.